I don't know if this has been posted before, but it's a very interesting topic:
The Four Horsemen:
The story behind both songs: "The Mechanix" is a heavy metal song written by Dave Mustaine, founder and lead singer of Megadeth. He originally wrote the song for Metallica, but Mustaine was later kicked out of the band right before they recorded their debut album, Kill 'Em All. The other members of the band changed the lyrics and added a new bridge section and released it on the album, retitling it "The Four Horsemen". Mustaine was still given credit for the song, although Mustaine claims he asked the band not to use his material, a charge Metallica disputes. The new lyrics refer to the Four Horsemen of the Apocalypse.
Mustaine formed Megadeth and released the song with its original lyrics a year later on Megadeth's debut album, Killing Is My Business... And Business Is Good!. The lyrics use several different automotive phrases and terms as sexual innuendo. This is the way the song was originally written (as can be heard on the early Metallica demo, No Life 'Til Leather), and lacks the bridge added to "The Four Horsemen." Megadeth's "The Mechanix" also has a much faster tempo than "The Four Horsemen."
Later, after forming Megadeth, Mustaine would sometimes perform the song in concert, differentiating between "their way" and "our way" of playing the song.